1. Field of the Invention
The present invention relates to the field of illumination apparatuses for planar display devices, and more particularly, to an illumination apparatus for planar display devices, the illumination apparatus making incident light travel parallel to an optical axis by reducing the emission angle of the incident light.
2. Description of the Related Art
Generally, a planar display device, such as a liquid crystal display, requires an illumination apparatus to uniformly illuminate liquid crystal.
FIG. 1 is a perspective view of a conventional illumination apparatus for planar display devices, the illumination apparatus including a light guide plate using a hologram The illumination apparatus of FIG. 1 uses dot light sources, such as laser emitting diodes (LEDs) 41.
Referring to FIG. 1, the plurality of LEDs 41 are aligned on one side of a light guide plate 43, and light emitted from the LEDs 41 is diffused at a predetermined angle and travel within the light guide plate 43. Since the light radiation angle of an LED 41 is typically about 45 degree, a dead space on which light is not incident is generated around the LEDs 41, and an overlapping area on which light beams radiated from adjacent LEDs 41 overlap each other and light intensity is increased is generated at a place distanced from the LEDs 41
If a hologram pattern 45 having a uniform directivity is formed on the light guide plate 43, light must be uniformly incident upon the hologram pattern 45, so that light is emitted with high luminous efficiency from the light guide plate 43 However, in the conventional light guide plate using dot light sources, such as the LEDs 41, light emitted from each of the dot light sources is diffused, such that the luminance distribution and intensity distribution depending on the angle of a light beam emitted from the light guide plate 43 are non-uniform.
FIG. 2 is a graph showing the luminous intensity according to the radiation angle of an LED As shown in FIG. 2, if the maximum of the luminous intensity at the center of an LED is 1, the luminous intensity decreases as the radiation angle increases. When the radiation angle is about 45 degrees, the light intensity is halved. It can be seen from FIG. 2 that the radiation angle needs to be narrowed to 45 degrees or less in order to reduce the loss of the luminous intensity.
FIG. 3 is a graph showing the distribution of luminance depending on the angle of light emitted from a light guide plate. FIG. 4 is a picture showing the distribution of intensity depending on the angle of light emitted from a conventional hologram light guide plate
Referring to FIG. 3, light is not emitted in the direction perpendicular to the plane of a light guide plate, and a large quantity of light is emitted at 0 degree and ±30 degrees, where luminance is peak. Light emitted at arbitrary angles other than a right angle with respect to the plane of a light guide plate causes loss of light incident upon a liquid crystal panel, thereby degrading luminance and affecting the viewing angle of a screen.
Referring to FIG. 4, the light intensity is high at zero and ±30 degrees, where luminance is peak as shown in FIG. 3.
In an illumination apparatus for planar display devices, using a light guide plate on which a conventional hologram is formed, light is emitted from dot light sources, such that light is incident upon the hologram at a non-uniform angle. Accordingly, the emission angle of light emitted from the light guide plate is the same as that of light emitted from each of the dot light sources. Thus, the luminance distribution and intensity distribution of light are not uniform.